Method and apparatus for increasing spatial resolution of a pet scanner

ABSTRACT

A method and apparatus for increasing the resolution of a Positron Emission Tomography scanner. The method and apparatus comprise elements and acts for centering a region of interest of an object at a point between first and second detector arrays which is at least about ten percent closer to the first detector array than to the second detector array.

[0001] This application is based upon and claims priority from a U.S.Provisional Application No. 60/394,135 filed on Jul. 5, 2002.

BACKGROUND OF THE INVENTION

[0002] The present invention relates generally to Positron EmissionTomography (“PET”) scanners, and more particularly to a method of PETscanning and a PET scanner apparatus having increased image resolution.

[0003] Positron Emission Tomography (“PET”) is an imaging technique thatprovides three-dimensional tomographic images of a distribution ofpositron-emitting isotopes within an object. The object is usually aliving human or animal, and the images provide a visual depiction oftissue differences within different portions of the object. A PETprocedure involves the introduction of radiolabeled tracingpharmaceuticals into the object, usually through injection orinhalation. The type of pharmaceuticals depends on the function of thetissue under investigation. As the radiolabeled tracing element in thepharmaceutical decays, it produces positrons. Each positron collideswith surrounding matter in the sample object before combining with anelectron in the sample object. The combination of each positron with anelectron in the sample object annihilates both particles, producing apair of gamma-ray photons. The gamma-ray photons travel away from theannihilation event in opposite directions. If a pair of opposinggamma-ray detectors each detect one of the two gamma-ray photonsproduced in the annihilation event within a predetermined period oftime, usually 5 to 50 nanoseconds, a “coincidence event” is recorded,and it is assumed that the annihilation event producing the gamma-rayphotons lies along a straight line between the two detectors.

[0004] Conventionally, a PET scanner consists of arrays of gamma-raydetectors, arranged either continuously as one or multiple rings, or astwo or more detector plates. Lines of response (“LOR”) are formedbetween opposing detector pairs in each array. The PET scanner obtainsthe radioactivity distribution information within the object bydetecting annihilation events originating along each LOR. Commerciallyavailable PET scanners having one or more rings of detectors areavailable for PET scanning animal and human subjects. The insidediameters of the one or more rings of detectors for PET scanning animaland human subjects are approximately 20 centimeters and 80 centimeters,respectively. Conventionally, for scanners having detectors arranged ina ring(s), the object is placed at the center of the ring(s) where thesampling is believed to be the highest, hence achieving the bestresolution and image quality currently available. For scanners havingdetector plates, the detector plates rotate around the object to collectdata from all angles in order to form a complete set of projections ofthe distribution. For the same reason as in the ring configuration, theobject is conventionally centered between the detector plates. Thedetection of a large number of annihilation events allows a computer toconstruct a three-dimensional image of the distribution of radiolabeledpharmaceuticals within the object, which provides valuable informationon the kinetics of the pharmaceuticals and functions of the livingobject.

[0005] With these conventional designs, the image spatial resolution ofa PET system is determined by several factors, including intrinsicdetector spatial resolution, acolinearity of the annihilating gamma rayphotons, and positron range of the radioisotopes in the tracingpharmaceuticals. Of these three factors, the last two depend on the typeof radioisotopes used and are independent of the scanner design.Therefore, PET scanner manufacturers have been trying to improve scannerspatial resolution by designing new detectors that improve thedetector's intrinsic spatial resolution. This is particularly importantfor very high resolution PET scanners dedicated to small animal imaging,which have become a very powerful tool for the advancement of molecularimaging.

[0006] For most animal PET scanners and some state-of-the-art humanscanners, discrete scintillation crystals coupled to photodetectors havebeen used to achieve the highest spatial resolution heretofore possible.For a PET scanner using discrete crystals, the detector intrinsicspatial resolution can not be better than one half of the crystal width.For a PET scanner with ring geometry, the detector pairs form samplinglines with an average sampling distance of half a crystal width. Basedon the Nyquist theorem in sampling theory, the smallest object (i.e.,the highest frequency of signal) that one system can resolve is twicethe size of the sampling distance (i.e., half of the samplingfrequency). In order to achieve image spatial resolution that approachesthe theoretical limit, where the detector intrinsic spatial resolutionequals one half the crystal width, conventional PET scanners requiresmaller sampling distances. Many attempts have been made to increase thesampling resolution. For example, certain designs move the detector orthe object by a fraction of the detector width. Other designs stackdiscrete crystals in multiple offset layers. With these designs, imageresolution can begin to approach the detector intrinsic resolution.However, conventional PET scanners have been unable to achieve imageresolution higher than the detector intrinsic spatial resolutionregardless of the type of gamma-ray detector employed. This is true forPET scanners with scintillation detectors, ionization chambers,semiconductor detectors and other types of gamma-ray detectors.

[0007] Several techniques have been developed in other imaging arts toresolve structures smaller than the detector intrinsic spatialresolution. One example is a gamma camera coupled to a pinholecollimator that produces a “magnified” image of the object, allowingimage resolution of objects smaller than the detector intrinsic spatialresolution. The drawback of this design is a significant reduction ofdetecting efficiency.

[0008] An example of another imaging device is a Compton camera havingtwo detectors placed to one side of a photon source. The detectors of aCompton camera are designed to sequentially detect a photon thatinteracts with one and then the other detector. The interaction with thefirst detector is through the Compton effect while the interaction withthe second detector is through the photoelectric effect. The sequentialdetection of a photon enables the Compton camera to trace the photon'spath without using mechanical collimators, such as those in a gammacamera. Therefore, a Compton camera has better sensitivity than theconventional gamma camera. The disadvantage of a Compton camera is itsreduced resolution compared with a conventional gamma camera. Incontrast to apparatus of the present invention, both detectors of theCompton camera are positioned on one side of a photon source and detecta single photon sequentially. In the present invention detectorspositioned in opposite sides of a photon source each detect separatephotons traveling in opposite direction from the coincidence event.Also, in contrast to the Compton camera, the present invention does notrely on Compton effect interactions to produce images of the object.

SUMMARY OF THE INVENTION

[0009] Briefly, the apparatus of this invention is a PET scanner forproviding an enhanced resolution image of a region of interest of anobject. The scanner comprises opposing first and second detector arraysspaced by a distance and a stage for holding the object between thefirst and second detector arrays. The detectors of the first detectorarray have an intrinsic spatial resolution that is equal to or betterthan the intrinsic spatial resolution of the detectors of the seconddetector array. The stage is located to center the region of interest ofthe object at a point between the first and second detector arrays. Thepoint is at least about ten percent closer to the first detector arraythan to the second detector array.

[0010] In another aspect of the invention, the point is at least aboutone centimeter closer to the first detector array than to the seconddetector array, and the distance between the two detector arrays isabout twenty centimeters.

[0011] In another aspect of the invention, the point is at least aboutfour centimeters closer to the first detector array than to the seconddetector array, and the distance between the two detector arrays isabout eighty centimeters.

[0012] In yet another aspect, the present invention is a PET scannercomprising opposing first and second detector arrays and a stage forholding the object between the first and second detector arrays. In thisaspect of the invention, the first and second detector arrays are eachformed as an arc of a circle, with the radius of the arc of the firstdetector being less than the radius of the arc of the second detectorarray. The detectors of the first detector array have an intrinsicspatial resolution that is equal to or better than the intrinsic spatialresolution of the detectors of the second detector array. The stage islocated to center the region of interest of the object at a pointbetween the first and second detector arrays. The point is at leastabout ten percent closer to the first detector array than to the seconddetector array.

[0013] In still another aspect, the present invention includes a scannercomprising a first circular detector array, a second circular detectorarray extending at least partially outside the first detector array, anda stage for holding the object inside the first and second detectorarrays. The detectors of the first detector array have an intrinsicspatial resolution that is equal to or better than the intrinsic spatialresolution of the detectors of the second detector array.

[0014] In yet another aspect, the present invention includes a methodfor increasing scanner resolution. The method comprises centering theregion of interest of the object at a point between the first and seconddetector arrays which is at least about ten percent closer to the firstdetector array than to the second detector array, and scanning theobject with the scanner.

[0015] In still another aspect of the invention, the region of interestof the object is centered at a point between the first and seconddetector arrays which is at least about one centimeter closer to thefirst detector array than to the second detector array, wherein thedistance between the two detector arrays is twenty centimeters.

[0016] In still another aspect of the invention, the region of interestof the object is centered at a point between the first and seconddetector arrays which is at least about four centimeters closer to thefirst detector array than to the second detector array, wherein thedistance between the two detector arrays is eighty centimeters.

[0017] In yet another aspect of the invention, a method of PET scanningis provided using a PET scanner comprising opposing first and seconddetector arrays that are each formed as an arc of a circle, with theradius of the arc of the first detector being less than the radius ofthe arc of the second detector array. The detectors of the firstdetector array have an intrinsic spatial resolution that is equal to orbetter than the intrinsic spatial resolution of the detectors of thesecond detector array. The method comprises centering the region ofinterest of the object at a point between the first and second detectorarrays which is at least about ten percent closer to the first detectorarray than to the second detector array, and scanning the object withthe scanner.

[0018] In another aspect, the present invention includes a method ofincreasing resolution of an image of a region of interest of an objectprovided by a positron emission tomography scanner comprising first andsecond circular concentric detector arrays. The detectors of the firstdetector array have an intrinsic spatial resolution that is equal to orbetter than the intrinsic spatial resolution of the detectors of thesecond detector array. The method comprises centering the region ofinterest of the object at a point inside the first and second detectorarrays and scanning the object with the scanner.

[0019] In yet another aspect the present invention includes animprovement in a PET scanner having a primary PET scanner for providingan image of a region of an object having opposing planar detector arraysspaced by a distance and include at least one detector having anintrinsic spatial resolution. The improvement comprises a PET detectormodule for providing an image of a region of an object having anaccessory planar detector array including at least one accessorydetector having an intrinsic spatial resolution at least as good as theintrinsic spatial resolution of the primary scanner detector. Theaccessory planar detector array is positioned inside an outer boundarydefined by the opposing planar detector arrays.

[0020] In a final aspect, the present invention includes an improvementin a PET scanner having a primary PET scanner for providing an image ofa region of an object having a primary detector array including aplurality of detectors defining an outer boundary and having anintrinsic spatial resolution. The improvement includes a PET detectormodule for providing an image of a region of an object having anaccessory detector array including a plurality of detectors. At leastone detector of the accessory detector array has an intrinsic spatialresolution at least as good as the intrinsic spatial resolution of eachof the primary scanner detectors. The accessory detector array ispositioned inside the outer boundary of the primary detector array.

[0021] Other objects of the present invention will be in part apparentand in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a schematic front elevation of a PET scanner having twoparallel detector arrays in accordance with a first embodiment of thepresent invention.

[0023]FIG. 2 is a second schematic front elevation of a PET scannerhaving two parallel detector arrays.

[0024] FIGS. 3A-3B are schematic front elevations of a PET scannerhaving two parallel detector arrays operating in accordance with asecond embodiment of the present invention.

[0025]FIG. 4 is schematic front elevation of a PET scanner having a ringof detectors operating in accordance with a third embodiment of thepresent invention.

[0026]FIG. 5 is a schematic front elevation of a PET scanner having aring of detectors operating in accordance with a fourth embodiment ofthe present invention.

[0027]FIG. 6 is a schematic front elevation of a PET scanner having aring of detectors operating in accordance with a fifth embodiment of thepresent invention.

[0028]FIG. 7A is a schematic front elevation of a PET scanner of a sixthembodiment of the present invention having two detector arrays arrangedin half rings of detectors having different radii.

[0029]FIG. 7B is a schematic front elevation of a PET scanner of aseventh embodiment of the present invention having two detector arraysarranged in full rings of detectors having different radii.

[0030] FIGS. 8A-8C are schematic front elevations of improved PETscanners having detector modules in accordance with eighth, ninth andtenth embodiments, respectively, of the present invention.

[0031] FIGS. 9A-9C are schematic vertical cross sections of improved PETscanner having a detector module in accordance with tenth, eleventh andtwelfth embodiments, respectively, of the present invention.

[0032]FIG. 10 is a perspective of a PET scanner in accordance with thepresent invention.

[0033]FIG. 11 is a schematic perspective of a PET scanner used for theexperimental results represented in FIGS. 12-14.

[0034] FIGS. 12A-12G are graphs illustrating the experimental resultsobtained using a conventional scanning method.

[0035] FIGS. 13A-13D are graphs illustrating the experimental resultsobtained using the scanning method of an embodiment of the presentinvention.

[0036] FIGS. 14A-14C are graphs illustrating the experimental resultsobtained using the scanning method of another embodiment of the presentinvention.

[0037] Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0038] Referring now to the drawings and in particular to FIG. 1, apositron emission tomography (“PET”) scanner is designated in itsentirety by the reference character 20. The scanner 20 provides an imageof a region of interest 22 of an object 24. The scanner 20 comprisesopposing first and second detector arrays 26, 28, respectively, spacedby a distance d and a stage 30 for holding the object 24 between thearrays. The first detector array 26 is formed from a series of adjacentgamma-ray photon detectors 31, while the second detector array 28 isformed from a series of adjacent gamma-ray photon detectors 32. Thedetectors 31 of the first detector array 26 have intrinsic spatialresolutions that are equal to or preferably better than intrinsicresolutions of the detectors 32 of the second detector array 28. Betterintrinsic spatial resolution, in other words resolution capable ofdistinguishing smaller structures of the object, may be achieved througha variety of known methods including, for example, decreasing the sizeof detectors 31. Detectors 31 and 32 may include scintillation crystals,ionization chambers, semiconductor material or other materials fordetecting gamma-ray photons. Detectors 31 and 32 may be of the same typeand the same size. Alternatively, detectors 31 and 32 may be of the sametype but different sizes, or of different types altogether. During PETscanning, the object 24 is positioned between the arrays 26 and 28 asdescribed below and radiolabeled tracing pharmaceuticals are introducedinto the object. As the radioisotopes in the tracing pharmaceuticalsdecay, they produce positrons. Each positron collides with surroundingmatter in the object and eventually combines with an electron andannihilates, producing a pair of gamma-ray photons. The gamma-rayphotons travel away from each other in a straight line (e.g., along line34). When the gamma-ray photons strike the detectors 31 and 32, they aredetected. If a pair of detectors 31 and 32 each detect one gamma-rayphoton within a predetermined period of time (e.g., within about 5 toabout 50 nanoseconds), it is assumed that the gamma-ray photons wereproduced at some point within the object along the straight lineextending between the two detectors (e.g., line 34). As will be apparentto those skilled in the art, a series of lines 40 (e.g., as illustratedin FIG. 2) extend between each pair of detectors 31 and 32 in the firstand second arrays 26 and 28. These lines 40 are referred to as “lines ofresponse,” and detector arrays 26 and 28 positioned on opposite sides ofthe object 24 are said to be in coincidence with one another. Photomultipliers and other electronic components (not shown) convertgamma-ray photon strikes into detection data and communicate thedetection data to a common coincidence processor such as a computer 64(FIG. 10). For example, scintillation detectors may be coupled tocommercially available photo-detectors such as Hamamatsu R8520U-C12 orto avalanche photo-multiplier coupled with optical fibers. The signalsfrom the photo-detectors or photo-multipliers may be amplified and thentransmitted to a coincidence processor. Other methods of convertinggamma-ray interactions into electronic detection data are well-known andmay also be used with this invention.

[0039] The position of the detector where the gamma-ray interactionoccurs is determined by a commonly used algorithm that defines the X andY coordinates of the interaction event. The coordinated are compared toa lookup table to identify the crystal, or other detector element wherethe gamma-ray interaction took place. The gamma-ray interaction event isencoded with the detector block number, the detector elementidentification within a block, the event energy, and the time of thedetection. This information makes up the detection data transmitted tothe coincidence processor and is used in standard image reconstruction.

[0040] As further illustrated in FIG. 1, the stage 30 is located betweenthe detector arrays 26 and 28 and is configured to hold the object 24 sothe region of interest 22 of the object is located at a specific point36 between the first and second detector arrays. In some embodiments ofthe invention, the point 36 where the region of interest 22 is held isat least about ten percent closer to the first detector array 26 than tothe second detector array 28. In other words, if the distance betweenthe region of interest 22 and the second detector array 28 is 100 units,the distance between the region of interest 22 and the first detectorarray 26 is no more than about 90 units. More preferably, the point 36is about 33% closer to the first array 26 than to the second array 28.Still more preferably, the point 36 is about 67% closer to the firstarray 26 than to the second array 28. Even more preferably, the point 36is about eighty percent closer to the first array 26 than to the secondarray 28. Preferably, the point 36 is no more than about 98% closer tothe first detector array 26 than to the second detector array 28 toavoid detector saturation.

[0041] Further, in some embodiments of the invention corresponding toPET scanning an animal subject, the point 36 where the region ofinterest 22 is held is at least about one centimeter closer to the firstdetector array 26 than to the second detector array 28, if the distanced between arrays 26 and 28 is about twenty centimeters. In otherembodiments of the invention corresponding to PET scanning a humansubject, the point 36 where the region of interest 22 is held is atleast about four centimeters closer to the first detector array 26 thanto the second detector array 28, if the distance d between arrays 26 and28 is eighty centimeters.

[0042] More preferably, the point 36 is between about four centimetersand about ten centimeters closer to the first array 26 than to thesecond array 28, if the distance d between arrays 26 and 28 is abouttwenty centimeters, corresponding to PET scanning an animal subject. ForPET scanning a human subject, the point 36 is preferably between aboutsixteen centimeters and about forty centimeters closer to the firstarray 26 than to the second array 28, if the distance d between arrays26 and 28 is about eighty centimeters.

[0043] Still more preferably, for PET scanning an animal subject, thepoint 36 is about fourteen centimeters closer to the first array 26 thanto the second array 28, if the distance d between arrays 26 and 28 isabout twenty centimeters. For PET scanning a human subject, the point 36is still more preferably about fifty-six centimeters closer to the firstarray 26 than to the second array 28, if the distance d between arrays26 and 28 is about eighty centimeters. For PET scanning an animalsubject, the point 36 is preferably located no closer than about onecentimeter to the first array 26, if the distance d between the arrays26 and 28 is about twenty centimeters. For PET scanning a human subject,the point 36 is preferably located no closer than about four centimetersto the first array 26, if the distance d between the arrays 26 and 28 isabout eighty centimeters.

[0044]FIG. 2 illustrates the advantages of centering the region ofinterest 22 of the object 24 as described above rather than asconventionally located (i.e., centered between the detector arrays 26and 28). As shown in FIG. 2, first and second detector arrays 26 and 28are positioned parallel to each other. Lines of response 40 are shownbetween individual detector pairs of the first and second detectorarrays 26 and 28, respectively. In the past, objects (not shown) werecentered between the arrays on a plane designated 42. The spacingbetween the lines of response 40 as they intersect this plane 42 isequal to half of the distance between adjacent detectors in arrays 26and 28. The best image spatial resolution achievable with this samplingdistance is approximately the size of the individual detectors. Usingconventional techniques that move the object or arrays 26 and 28 a smalldistance, e.g., one-half or one-quarter of the detector width, the imageresolution can approach the theoretical limit of the conventionaldesigns, i.e., the detector intrinsic spatial resolution. By positioningthe object closer to the first array 26 than to the second array 28,e.g., on a plane designated 44, and scanning the object, the resolutionis improved. The improvement in resolution is represented by twoeffects: improved sampling and magnified projection. The spacing betweenthe adjacent lines of response 40 as they intersect the plane 44 isreduced compared to the spacing between the adjacent lines of response40 as they intersect the plane 42. For example, an object placed on aplane 67% closer to array 26 than to array 28 will produce a samplingresolution twice as fine as an object centered between the arrays.Although the usable imaging area of an object placed on plane 44 issmaller than for an object placed on plane 42, the region of interest 22(as in FIG. 1) of the object 24 (as in FIG. 1) on plane 44 is projectedonto the farther detector array 28 with greater magnification comparedto conventional techniques. For example, an object placed on a plane 67%closer to array 26 than to array 28 will produce a magnification of twotimes larger at the farther array 28 compared to an object centeredbetween the arrays. Thus, as will be appreciated by those skilled in theart, centering the region of interest 22 (as in FIG. 1) of the object 24(as in FIG. 1) at a point closer to the first detector array 26 than tothe second detector array 28 can produce an image with spatialresolution higher than the intrinsic resolution of individual detectorsin array 28.

[0045] The resolution of the scanner may be improved by moving theobject relative to the detector arrays or moving the arrays relative tothe object. As will be appreciated by those skilled in the art, movingthe object or arrays reduces the effective sampling size.

[0046] In order to produce a 3-dimensional tomographic image,projections of the object from multiple perspectives are required. Toobtain the magnified projections of the object from multipleperspectives, one can either rotate the detector arrays 26 and 28 aboutthe stationary object 24, or rotate the object 24 and keep the detectorarrays 26 and 28 stationary. The goal is to acquire the magnifiedprojections of the object with finer sampling size from multipleperspectives, which will result in PET images with enhanced spatialresolution.

[0047] One example of this resolution improvement technique isillustrated in FIG. 3A. An object 24 is illustrated centered about astationary axis 46 and positioned between a first detector array 26 anda second detector array 28 as described above. FIG. 3B illustratesseveral successive positions of the apparatus achieved by rotating thefirst detector array 26 and the second detector array 28 about the axis46. The detector arrays 26 and 28 are rotated, either continuously whilescanning the object or in discrete steps, in order to produce magnifiedprojections of the object from multiple perspectives.

[0048]FIG. 4 illustrates the first and second detector arrays arrangedas a continuous ring detector array 60 having opposing first and seconddetector arrays (not shown), formed as arcs or halves, separated by adistance d as shown in FIG. 5. The arcs constitute the first or thesecond detector array. The detectors of the first detector array haveintrinsic spatial resolutions that are equal to or preferably betterthan intrinsic spatial resolutions of the detectors of the seconddetector array. An object 24 is illustrated centered about a stationaryaxis 46 and arranged inside the ring detector array 60 at a point closerto the first detector array than the second detector array to achievethe desired magnification effect. The object is scanned while the ringdetector array 60 is rotated about the axis 46 either continuously whilescanning the object or in discrete steps. One of several successivepositions of the apparatus as the ring detector array 60 is rotatedabout the axis 46 is designated by reference numeral 62 a. During therotation, the first detector array is positioned closer to the objectthan the second detector array. The high resolution images require onlythe data from the individual detector pairs that produce a magnificationof the object. Therefore, the raw data from all individual detectorpairs is analyzed using a computer 64 (FIG. 8) to extract the datacorresponding to the magnified projections of the object from multipleperspectives.

[0049]FIG. 5 illustrates a ring detector array 70 as previouslydescribed with reference to FIG. 4, with an object 24 centered about anaxis 46. The object 24 is arranged inside the ring detector array 70 ata point closer to the first detector array than the second detectorarray to achieve the desired magnification effect. In this embodiment,the ring detector array 70 remains stationary while the object isrotated, either continuously or in discrete steps, about the axis 46 andscanned with the ring detector array 70. One of the several successivepositions of the object 24 as it is rotated about the axis 46 isdesignated by the reference numeral 72 a. During the rotation, theobject is positioned closer to the first detector array than the seconddetector array. The high resolution images require only the data fromthe individual detector pairs that produce a magnification of theobject. Therefore, the raw data from all individual detector pairs isanalyzed using a computer 64 (FIG. 10) to extract the data correspondingto the magnified projections of the object from multiple perspectives.

[0050]FIG. 6 illustrates a ring detector array 80 as previouslydescribed with reference to FIG. 4, having a central axis 82 and anobject 24 arranged inside the ring detector array at a point closer tothe first detector array than the second detector array to achieve thedesired magnification effect. In this embodiment, the object 24 orbitseither continuously or in discrete steps, about the axis 82 at apredetermined angular rate as it is scanned by the ring detector array80. The ring detector array 80 rotates about the axis 82 at the sameangular rate as the object 24 so the position of object 24 remainscloser to the first detector array than the second detector array. Oneof the several successive positions of the object 24 as it orbits aboutthe axis 82 is designated by the reference numeral 84 a. The highresolution images require only the data from the individual detectorpairs that produce a magnification of the object. Therefore, the rawdata from all individual detector pairs is analyzed using a computer 64(FIG. 10) to extract the data corresponding to the magnified projectionsof the object from multiple perspectives.

[0051]FIG. 7A illustrates a PET scanner comprising a first detectorarray 26 and an opposing second detector array 28 each shaped in theform of a half circle having different radii and each centered about acommon axis 90. The detectors of the first detector array 26 haveintrinsic spatial resolutions equal to or better than intrinsic spatialresolutions of the detectors of the second detector array 28. An object24 is centered on the axis 90 inside the first detector array 26 and theopposing detector array 28. Lines of response 40 demonstrate that theimage of the object projected onto the second detector array 28 ismagnified, thus achieving higher resolution. The design of FIG. 7Aachieves high resolution images while the first detector array 26 andthe second detector array 28 rotate about the axis 90 to producemagnified projections of the object from multiple perspectives.Alternatively, the first detector array 26 and the second detector array28 can remain stationary and the PET scanner can be used to acquireprojections of the object from multiple perspectives at the same time.Although the alternative does not produce magnified projections of theobject from all angles, its image resolution is still equal to orbetter? than conventional designs. This embodiment with stationarydetector arrays allows dynamic scanning of the object which is importantin some PET applications.

[0052]FIG. 7B illustrates a PET scanner comprising a first detectorarray 26 and a second detector array 28 each shaped in the form of afull circle having different radii and each centered about a common axis90. The detectors of the first detector array 26 have intrinsic spatialresolutions equal to or better than intrinsic spatial resolutions of thedetectors of the second detector array 28. An object 24 is centered onthe axis 90 inside the first detector array 26 and the second detectorarray 28. Because gamma rays carry high energy and can penetratematerials, some gamma rays originating from the object 24 pass throughthe inner detector array 26 without interaction and are detected by theouter detector array 28. Therefore, lines of response can extend betweenthe first (inner) detector array 26 and the second (outer) detectorarray 28 (designated by 40), or between opposing halves of the innerdetector array 26 (designated by 39), and between opposing halves of theouter detector array 28 (designated by 41). Because the lower half ofthe inner detector array 26 and the upper half of the outer detectorarray 28 resemble the design in FIG. 7A, the scanner shown in FIG. 7Bcan be thought of as two scanners such as shown in FIG. 7A beingcombined together. There are at least two advantages to theconfiguration shown in FIG. 7B. First, it eliminates the moving parts.Second, it acquires both “conventional PET images” and “high resolutionimages” at the same time. Data collected from lines of response 39 and41 can be used to reconstruct PET images with conventional resolution,while data from lines of response 40 can provide high resolution imagesas described above.

[0053] Those skilled in the art will appreciate that the inventiondisclosed herein can be adapted to improve existing, commerciallyavailable conventional PET scanners.

[0054]FIG. 8A illustrates an embodiment of an improved PET scannercomprising a primary scanner having a circular detector array 48. Thecircular detector array 48 encircles a cylindrical volume (having acylindrical outer boundary and planar ends (designated by lines 56 and58 in FIGS. 9A-9C) and include a plurality of detectors (not shown)having intrinsic spatial resolutions of about 3 to about 15 millimeters.The improvement comprises at least one secondary PET detector modulepositioned inside the cylindrical boundary of the primary PET scanner,and whose imaging data can be combined with that of the primary PETscanner using a computer 64 (FIG. 10). The secondary PET detector modulecomprises at least one accessory detector array 50, configured forexample in the form of a half ring and having one or more accessorydetectors (not shown). The intrinsic spatial resolutions of theaccessory detectors are equal to or better than the intrinsic spatialresolutions of the detectors of the circular detector arrays 48. Theaccessory detector array 50 can be positioned close to the object 24.Lines of response 40 extend from the accessory detector array 50 to thecircular detector arrays 48 and demonstrate that the image of the objectprojected onto the circular detector array 48 is magnified, thusachieving higher resolution. The design of FIG. 8A achieves highresolution images while the accessory detector array 50 rotates aroundthe object 24 to produce magnified projections of the object frommultiple perspectives. Alternatively, the accessory detector array 50can remain stationary and the secondary PET detector module can be usedto acquire projections of the object from multiple perspectives at thesame time.

[0055] Alternatively, a person skilled in the art will appreciate thatthe magnification effects achieved with the secondary PET detectormodule illustrated in FIG. 8A can be achieved using planar detectorarrays.

[0056]FIG. 8B illustrates an improved PET scanner having at least onecircular detector array 48. The circular detector arrays 48 include aplurality of detectors (not shown) having intrinsic spatial resolutionsof about 3 to about 15 mm. The improvement can be used for, among otherthings, imaging breast tissue and comprises a secondary PET detectormodule having a plurality of accessory detector arrays 52. The secondaryPET detector module is arranged so that the accessory detector arrays 52are positioned inside the cylindrical boundary of the primary PETscanner. In this embodiment, the accessory detector arrays 52 are formedas adjacent arcs configured to receive human breasts. Each arc has aplurality of accessory detectors (not shown). The intrinsic resolutionsof the accessory detectors are equal to or better than the intrinsicresolutions of the detectors of the circular detectors array 48. Linesof response 40 extend from each of the accessory detector arrays 52 tothe circular detector array 48 and demonstrate that the image of theobject projected onto the circular detector arrays 48 is magnified, thusachieving higher resolution. The plurality of accessory detector arrays52 can remain stationary and the secondary PET detector module can beused to acquire simultaneous projections of the object from multipleperspectives.

[0057]FIG. 8C illustrates an improved PET scanner having at least onecircular detector array 48. The array 48 includes a plurality ofdetectors (not shown) that have intrinsic spatial resolutions of about 3mm to about 15 mm. The improvement comprises a secondary PET detectormodule positioned inside the cylindrical boundary of the primary PETscanner, and whose data can be combined with that of the primary PETscanner using a computer 64 (FIG. 10). The secondary PET detector moduleincludes at least one accessory detector array 54, configured forexample as a ring detector, having at least one accessory detector (notshown). The intrinsic spatial resolutions of the accessory detectors isequal to or better than the intrinsic spatial resolution of thedetectors of the primary detector array 48. The design of FIG. 8Cachieves high resolution while the circular-detector array 48 and theaccessory detector arrays 54 remain stationary. Lines of response 40extend from the accessory detector array 54 to the circular detectorarrays 48 and demonstrate that the image of the object projected ontothe circular detector arrays 48 is magnified, thus achieving higherresolution. Lines of response 41 extend from and to opposing sides ofthe circular detector arrays 48 demonstrating that images of the objectwith conventional intrinsic spatial resolution may be obtained.Alternatively, the design can capture enhanced high resolution images asshown with lines of response 39 extending from and to the accessorydetectors of the accessory detector arrays 54.

[0058] As previously described, the accessory detector arrays 54 arepreferably positioned inside the outer boundary formed by the primaryscanner. However, as illustrated in FIGS. 9A-9C, the accessory detectorarrays 54 can be positioned between or entirely outside the planar endsof the primary scanner designated by lines 56 and 58. Because lines ofresponse extend in all directions, coincidence events between theaccessory detector arrays 54 and the primary detector array 48 can becaptured and the image of an object reconstructed using the computer 64(FIG. 10) and standard image reconstruction techniques.

[0059] As will be apparent to a person skilled in the art, detectorarrays 48 and the accessory detector arrays may have variousconfigurations and may have various positions with respect to each otherwhile still achieving the benefits of the present invention.

[0060] Integration of the detection data from the detector module withthe detection data from the PET scanner may be accomplished using someof the data channels of the PET scanner. For example, the channels usedfor transmitting detection data from one of the circular detector arrays48 may be rerouted and configured using standard electronic techniquesfor transmitting detection data from the PET detector module to thecoincidence processor. The coincidence processor compares the detectiondata from the PET detector module with the detection data from the PETscanner to determine the time and position of coincidence events.

[0061] In those embodiments that include a PET detector module (FIGS.8A-8C and 9A-9C), the detection data processing scheme will involvethree sets of data. The first set of data includes the coincidenceevents detected by the accessory detectors of the accessory detectorarrays 50, 52 or 54 and the circular detector arrays 48. The second setof data includes the coincident events detected by accessory detectorsof the accessory detector arrays 50, 52 and 54. The third set of dataincludes the coincidence events detected by the detectors of thecircular detector array 48. These data sets are registered in a fullthree-dimensional sinogram. The sinograms are used to reconstruct animage of the object using standard imaging techniques.

[0062]FIG. 10 illustrates a PET scanner 20 comprising a ring detectorarray 80 mounted on a gantry 110. A patient bed 114 having a stage 116is shown slidably mounted on a pedestal 118. The ring detector array 80comprises detectors (not shown). A computer 64 is provided to controlthe ring detector array 80, the patient bed 114, and the pedestal 118.The computer 64 also collects and analyzes the data from the ringdetector array 80.

[0063] The apparatus or method of the present invention adds versatilityto PET scanners that can be used in several modes. If the object ispositioned in the center of a PET scanner 20 (not shown), images can beacquired conventionally. If the object is positioned as illustrated inFIG. 10, the system is in high-resolution mode. This mode allows one to“zoom in” on the object to view the region of interest with higherresolution. Alternatively, the ring detector array 80 may have detectors(not shown) having different intrinsic spatial resolutions. Bypositioning the patient bed 114 and the object closer to those detectorsthat have higher intrinsic spatial resolution, an enhancedhigh-resolution mode is achieved. Existing PET scanners such as ART orHR-plus scanners available from CTI/Siemens AG, Advantage scannersavailable from General Electric Company, Allegro scanners available fromRoyal Philips Electronics, microPET scanners available from ConcordeMicrosystems Inc. and others may be adapted to operate in accordancewith the invention as illustrated in FIGS. 8A-8C. Also, future PETscanner designs may benefit from this invention.

[0064] An experimental PET scanner arrangement illustrated in FIG. 11has been successfully tested in the laboratory. The results areillustrated in FIGS. 12-14.

[0065]FIG. 11 illustrates the PET scanner used for performing theexperiments. The PET scanner consisted of an upper detector array 28consisting of 14×14 scintillation detectors 32 and a lower detector“array” 25 consisting of a single scintillation detector. Thescintillation detectors were made of lutetium oxyorthosilicate (LSO)crystals. The crystals were individually cut to 1×1×12.5 mm³ andpolished to optical grade for optimal light collection. The crystalpitch (i.e., center-to-center distance between adjacent crystals) was1.15 mm to accommodate the reflective material inserted between thecrystals for optical isolation purposes. The entire array 28 measured16.1×16.1×12.5 mm³. The photodetectors behind these scintillationcrystals were Hamamatsu H7546 64-channel photomultiplier tubes.

[0066] The separation between the upper and lower detector arrays wasfixed at a total distance D of 12.5 cm during the entire series oftests. The positron-emitting sample 27 in these experiments was a ²²Napoint source approximately 0.6 mm in diameter, embedded in a plasticcasing. In the first test representing conventional PET scanningmethods, the sample 27 was stepped through the imaging field of viewalong a plane 42 with a step size of 1.0 mm. Coincidence events betweenthe upper and lower detector arrays were recorded for five minutes ateach location. Each of the 196 scintillating detectors on the upperdetector array 28 formed a line of response 40 with the lower detector“array” 25 consisting of a single scintillating detector. If a line ofresponse 40 passed through the sample 27, it detected the annihilatingevent. The number of annihilating events detected by the individualscintillating detectors 32 of the upper detector array 28, incoincidence with the scintillating detector array 25, were sorted into a2-dimensional histogram presented as surface plots in FIGS. 12A-12G. Asthe point sample 27 stepped through the central plane, the peak of thedetected events moved from one side of the plots in FIGS. 12A-12G to theother. In the first test, it took more than 7 steps to move the sample27 out of the detectable Field of View, indicating a minimum of 8×8 mm²usable imaging Field of View.

[0067] In the second test representing an embodiment of the presentinvention, the sample 27 was moved to plane 44, ¼ of the total distanceD (3.1 cm) from the lower detector “array” 25, and ¾ of the totaldistance D (9.4 cm) from the upper detector array 28. Data was collectedand sorted into a 2-dimensional histogram presented as surface plots inFIGS. 13A-13D. With the same step size of 1.0 mm as in test 1, it tookonly 4 steps to move the sample 27 out of the Field of View, indicatingthat the usable imaging Filed of View was around 4×4 mm². The surfaceplots of FIGS. 13A-13D also illustrate that the projection of the pointsource covers a much larger surface area of the detector array,demonstrating the magnification effect of the present invention.

[0068] In the final test (not shown in FIG. 11) representing anotherembodiment of the present invention, the sample 27 was moved to ⅕ of thetotal distance D (2.5 cm) from the lower detector “array” 25, and ⅘ ofthe total distance D (10 cm) from the upper detector array 28. Data wascollected and sorted into a 2-dimensional histogram presented as surfaceplots in FIGS. 14A-14C. It only took three steps to cover the entireField of View, indicating an even smaller usable imaging Field of Viewof around 3×3 mm². The surface plots of FIGS. 14A-14C show that theprojection of the point source covers an even larger surface area of thedetector array, demonstrating the magnification effect of the presentinvention.

[0069] The detected annihilation event count rate between the upper andlower detector arrays 28 and 25 was found to be 30 cps (“counts persecond”), 120 cps, and 180 cps for tests 1, 2 and 3, respectively. Thereason that the detection efficiency increased 4 and 6 times in test 2and 3, respectively, compared to test 1, was due to the small sample 27distribution and a single scintillating detector making up the lowerdetector “array” 25. All coincidence events that were detected by thelower detector “array” 25 fell completely within the surface area of theupper detector array 28 in all three configurations. Therefore, therewas no detection efficiency loss for the upper detector array 28 whenthe sample 27 was moved away from the upper detector array 28. Thecoincidence detecting efficiency gain was therefore the same as theefficiency gain of the lower detector “array” 25. For a scanner that hasa much larger number of scintillating detectors 32 both at the upper andlower detector arrays, the improvement in detecting efficiency would bereduced to approximately 77% and 144% for the configurations in test 2and 3, compared to the configuration in test 1.

[0070] In view of the above, it will be seen that the several objects ofthe invention are achieved and other advantageous results attained.

[0071] When introducing elements of the present invention or thepreferred embodiment(s) thereof, the articles “a”, “an”, “the” and“said” are intended to mean that there are one or more of the elements.The terms “comprising”, “including” and “having” are intended to beinclusive and mean that there may be additional elements other than thelisted elements.

[0072] As various changes could be made in the above constructionswithout departing from the scope of the invention, it is intended thatall matter contained in the above description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. A method of increasing resolution of an image ofa region of interest of an object provided by a positron emissiontomography scanner comprising opposing first and second detector arraysspaced by a distance, said method comprising the steps of: centering theregion of interest of the object at a point between the first and seconddetector arrays which is at least about ten percent closer to said firstdetector array than to said second detector array; and scanning theobject with the scanner.
 2. A method as set forth in claim 1 wherein thepoint is at least about thirty-three percent closer to said firstdetector array than to said second detector array.
 3. A method as setforth in claim 2 wherein the point is at least about sixty-seven percentcloser to said first detector array than to said second detector array.4. A method as set forth in claim 3 wherein the point is abouteighty-two percent closer to said first detector array than to saidsecond detector array.
 5. A method as set forth in claim 1 wherein thepoint is no more than about ninety-eight percent closer to said firstdetector array than to said second detector array.
 6. A method as setforth in claim 1 further comprising the steps of: moving said objectrelative to said first and second detector arrays; and rescanning theobject with the scanner.
 7. A method as set forth in claim 6 wherein theobject is rescanned after being moved.
 8. A method as set forth in claim6 wherein the object is rescanned while being moved.
 9. A method as setforth in claim 6 wherein the object is rotated relative to said firstand second detector arrays.
 10. A method as set forth in claim 9 whereinthe object is rotated through an angle of between about zero degrees andabout 360 degrees.
 11. A method as set forth in claim 1 furthercomprising the steps of: moving at least one of said first and seconddetector arrays relative to said object; and rescanning the object withthe scanner.
 12. A method as set forth in claim 11 wherein the step ofmoving the arrays comprises rotating said first and second detectorarrays about an axis.
 13. A method as set forth in claim 12 wherein theaxis is closer to the first array than to said second array.
 14. Amethod as set forth in claim 13 wherein the axis extends through theregion of interest of the object.
 15. A method as set forth in claim 12wherein said first and second arrays are rotated through an angle ofbetween about zero degrees and about 360 degrees.
 16. A method ofincreasing resolution of an image of a region of interest of an objectprovided by a positron emission tomography scanner comprising opposingfirst and second detector arrays spaced by no more than about twentycentimeters, said method comprising the steps of: centering the regionof interest of the object at a point between the first and seconddetector arrays which is at least about one centimeter closer to saidfirst detector array than to said second detector array; and scanningthe object with the scanner.
 17. A method as set forth in claim 16wherein the point is at least about five centimeters closer to saidfirst detector array than to said second detector array.
 18. A method asset forth in claim 17 wherein the point is at least about tencentimeters closer to said first detector array than to said seconddetector array.
 19. A method as set forth in claim 18 wherein the pointis about fourteen centimeters closer to said first detector array thanto said second detector array.
 20. A method as set forth in claim 16wherein the point is no closer than about one centimeter to said firstdetector array.
 21. A method as set forth in claim 16 further comprisingthe steps of: moving said object relative to said first and seconddetector arrays; and rescanning the object with the scanner.
 22. Amethod as set forth in claim 21 wherein the object is rotated relativeto said first and second detector arrays.
 23. A method as set forth inclaim 22 wherein the object is rotated through an angle of between aboutzero degrees and about 360 degrees.
 24. A method as set forth in claim16 further comprising the steps of: moving at least one of said firstand second detector arrays relative to said object; and rescanning theobject with the scanner.
 25. A method as set forth in claim 24 whereinthe step of moving the arrays comprises rotating said first and seconddetector arrays about an axis.
 26. A method as set forth in claim 25wherein the axis is closer to the first array than to said second array.27. A method as set forth in claim 26 wherein the axis extends throughthe region of interest of the object.
 28. A method as set forth in claim25 wherein said first and second arrays are rotated through an angle ofbetween about zero degrees and about 360 degrees.
 29. A method ofincreasing resolution of an image of a region of interest of an objectprovided by a positron emission tomography scanner comprising opposingfirst and second detector arrays spaced by no more than about eightycentimeters, said method comprising the steps of: centering the regionof interest of the object at a point between the first and seconddetector arrays which is at least about four centimeters closer to saidfirst detector array than to said second detector array; and scanningthe object with the scanner.
 30. A method as set forth in claim 29wherein the point is at least about twenty centimeters closer to saidfirst detector array than to said second detector array.
 31. A method asset forth in claim 30 wherein the point is at least about fortycentimeters closer to said first detector array than to said seconddetector array.
 32. A method as set forth in claim 31 wherein the pointis about fifty-six centimeters closer to said first detector array thanto said second detector array.
 33. A method as set forth in claim 29wherein the point is no closer than about four centimeters to said firstdetector array.
 34. A method as set forth in claim 29 further comprisingthe steps of: moving said object relative to said first and seconddetector arrays; and rescanning the object with the scanner.
 35. Amethod as set forth in claim 34 wherein the object is rotated relativeto said first and second detector arrays.
 36. A method as set forth inclaim 35 wherein the object is rotated through an angle of between aboutzero degrees and about 360 degrees.
 37. A method as set forth in claim29 further comprising the steps of: moving at least one of said firstand second detector arrays relative to said object; and rescanning theobject with the scanner.
 38. A method as set forth in claim 37 whereinthe step of moving the arrays comprises rotating said first and seconddetector arrays about an axis.
 39. A method as set forth in claim 38wherein the axis is closer to the first array than to said second array.40. A method as set forth in claim 39 wherein the axis extends throughthe region of interest of the object.
 41. A method as set forth in claim38 wherein said first and second arrays are rotated through an angle ofbetween about zero degrees and about 360 degrees.
 42. A positronemission tomography scanner for providing an image of a region ofinterest of an object, said scanner comprising: opposing first andsecond detector arrays spaced by a distance; a stage for holding theobject between said first and second detector arrays, said stage beinglocated to center the region of interest of the object at a pointbetween the first and second detector arrays; wherein said point islocated at least about ten percent closer to said first detector arraythan to said second detector array.
 43. A positron emission tomographyscanner as set forth in claim 42 wherein each of said first and seconddetector arrays has at least one detector having an intrinsic spatialresolution, and wherein the intrinsic spatial resolution of at least oneof the detectors of said first detector array is at least as great asthe intrinsic spatial resolution of at least one of the detectors ofsaid second detector array.
 44. A positron emission tomography scanneras set forth in claim 43 wherein the intrinsic spatial resolution ofeach detector of the first detector array is better than the intrinsicspatial resolution of each detector of the second detector array.
 45. Apositron emission tomography scanner as set forth in claim 42 whereinthe point is located at least about thirty-three percent closer to saidfirst detector array than to said second detector array.
 46. A positronemission tomography scanner as set forth in claim 43 wherein the pointis located at least about sixty-seven percent closer to said firstdetector array than to said second detector array.
 47. A positronemission tomography scanner as set forth in claim 46 wherein the pointis located about eighty-two percent closer to said first detector arraythan to said second detector array.
 48. A positron emission tomographyscanner as set forth in claim 42 wherein the point is located no morethan about ninety-eight percent closer to said first detector array thanto said second detector array.
 49. A positron emission tomographyscanner for providing an image of a region of interest of an object,said scanner comprising: opposing first and second detector arraysspaced by no more than about twenty centimeters; a stage for holding theobject between said first and second detector arrays, said stage beinglocated to center the region of interest of the object at a pointbetween the first and second detector arrays, said point being at leastabout one centimeter closer to said first detector array than to saidsecond detector array.
 50. A positron emission tomography scanner as setforth in claim 49 wherein each of said first and second detector arrayshas at least one detector having an intrinsic spatial resolution, andwherein the intrinsic spatial resolution of at least one of thedetectors of said first detector array is at least as good as theintrinsic spatial resolution of at least one of the detectors of saidsecond detector array.
 51. A positron emission tomography scanner as setforth in claim 50 wherein the intrinsic spatial resolution of eachdetector of the first detector array is better than the intrinsicspatial resolution of each detector of the second detector array.
 52. Apositron emission tomography scanner as set forth in claim 47 whereinsaid point is located at least about five centimeters closer to saidfirst detector array than to said second detector array.
 53. A positronemission tomography scanner as set forth in claim 52 wherein said pointis located at least about ten centimeters closer to said first detectorarray than to said second detector array.
 54. A position emissiontomography scanner as set forth in claim 53 wherein the point is locatedabout fourteen centimeters closer to said first detector array than tosaid second detector array.
 55. A positron emission tomography scanneras set forth in claim 49 wherein said point no closer than about onecentimeter to said first detector array.
 56. A positron emissiontomography scanner for providing an image of a region of interest of anobject, said scanner comprising: opposing first and second detectorarrays spaced by no more than about eighty centimeters; a stage forholding the object between said first and second detector arrays, saidstage being located to center the region of interest of the object at apoint between the first and second detector arrays, said point being atleast about four centimeters closer to said first detector array than tosaid second detector array.
 57. A positron emission tomography scanneras set forth in claim 52 wherein each of said first and second detectorarrays has at least one detector having an intrinsic spatial resolution,and wherein the intrinsic spatial resolution of at least one of thedetectors of said first detector array is at least as good as theintrinsic spatial resolution of at least one of the detectors of saidsecond detector array.
 58. A positron emission tomography scanner as setforth in claim 57 wherein the intrinsic spatial resolution of eachdetector of the first detector array is better than the intrinsicspatial resolution of each detector of the second detector array.
 59. Apositron emission tomography scanner as set forth in claim 56 whereinsaid point is located at least about twenty centimeters closer to saidfirst detector array than to said second detector array.
 60. A positronemission tomography scanner as set forth in claim 59 wherein said pointis located at least about forty centimeters closer to said firstdetector array than to said second detector array.
 61. A positronemission tomography scanner as set forth in claim 60 wherein the pointis located about fifty-six centimeters closer to said first detectorarray than to said second detector array.
 62. A positron emissiontomography scanner as set forth in claim 56 wherein said point islocated no closer than about four centimeters to said first detectorarray.
 63. A positron emission tomography scanner for providing an imageof a region of interest of an object, said scanner comprising: opposingfirst and second detector arrays, each of said first and second detectorarrays being formed as an arc of a circle, wherein the radius of the arcof the first detector array is less than the radius of the arc of thesecond detector array; a stage for holding the object between said firstand second detector arrays, said stage being located to center theregion of interest of the object at a point between the first and seconddetector arrays, said point being at least about ten percent closer tosaid first detector array than to said second detector array.
 64. Apositron emission tomography scanner as set forth in claim 63 whereineach of said first and second detector arrays has at least one detectorhaving an intrinsic spatial resolution, and wherein the intrinsicspatial resolution of at least one of the detectors of said firstdetector array is at least as good as the intrinsic spatial resolutionof at least one of the detectors of said second detector array.
 65. Apositron emission tomography scanner as set forth in claim 64 whereinthe intrinsic spatial resolution of each detector of the first detectorarray is better than the intrinsic spatial resolution of each detectorof the second detector array.
 66. A positron emission tomography scanneras set forth in claim 63 wherein each of said first and second detectorarrays are formed as a half circle.
 67. A positron emission tomographyscanner as set forth in claim 63 wherein the arc of the first detectorarray and the arc of the second detector array are centered about acommon axis.
 68. A positron emission tomography scanner as set forth inclaim 63 wherein the point is at least about thirty-three percent closerto said first detector array than to said second detector array.
 69. Apositron emission tomography scanner as set forth in claim 68 whereinthe point is at least about sixty-seven percent closer to said firstdetector array than to said second detector array.
 70. A positronemission tomography scanner as set forth in claim 69 wherein the pointis about eighty-two percent closer to said first detector array than tosaid second detector array.
 71. A positron emission tomography scanneras set forth in claim 63 wherein the point is no more than aboutninety-eight percent closer to said first detector array than to saidsecond detector array.
 72. A method of increasing resolution of an imageof a region of interest of an object provided by a positron emissiontomography scanner comprising opposing first and second detector arrays,said first and second detector arrays each being formed as an arc of acircle, wherein the radius of the circle of the arc of the firstdetector array is less than the radius of the circle of the arc of thesecond detector array, said method comprising the steps of: centeringthe region of interest of the object at a point between the first andsecond detector arrays which is at least about ten percent closer tosaid first detector array than to said second detector array; andscanning the object with the scanner.
 73. A positron emission tomographyscanner as set forth in claim 72 wherein each of said first and seconddetector arrays has at least one detector having an intrinsic spatialresolution, and wherein the intrinsic spatial resolution of at least oneof the detectors of said first detector array is at least as good as theintrinsic spatial resolution of at least one of the detectors of saidsecond detector array.
 74. A positron emission tomography scanner as setforth in claim 73 wherein the intrinsic spatial resolution of eachdetector of the first detector array is better than the intrinsicspatial resolution of each detector of the second detector array.
 75. Amethod as set forth in claim 72 wherein the point is at least aboutthirty-three percent closer to said first detector array than to saidsecond detector array.
 76. A method as set forth in claim 75 wherein thepoint is at least about sixty-seven percent closer to said firstdetector array than to said second detector array.
 77. A method as setforth in claim 76 wherein the point is about eighty-two percent closerto said first detector array than to said second detector array.
 78. Amethod as set forth in claim 72 wherein the point is no more than aboutninety-eight percent closer to said first detector array than to saidsecond detector array.
 79. A method as set forth in claim 72 furthercomprising the steps of: moving said object relative to said first andsecond detector arrays; and rescanning the object with the scanner. 80.A method as set forth in claim 79 wherein the object is scanned afterbeing moved.
 81. A method as set forth in claim 79 wherein the object isscanned while being moved.
 82. A method as set forth in claim 79 whereinthe object is rotated relative to said first and second detector arrays.83. A method as set forth in claim 82 wherein the object is rotatedthrough an angle of between about zero degrees and about 360 degrees.84. A method as set forth in claim 72 further comprising the steps of:moving at least one of said first and second detector arrays relative tosaid object; and rescanning the object with the scanner.
 85. A method asset forth in claim 84 wherein the step of moving the arrays comprisesrotating said first and second detector arrays about an axis.
 86. Amethod as set forth in claim 85 wherein the axis is closer to the firstarray than to said second array.
 87. A method as set forth in claim 86wherein the axis extends through the region of interest of the object.88. A method as set forth in claim 85 wherein said first and secondarrays are rotated through an angle of between about zero degrees andabout 360 degrees.
 89. A positron emission tomography scanner forproviding an image of a region of interest of an object, said scannercomprising: a first circular detector array; a second circular detectorarray concentric with said first detector array; and a stage for holdingthe object inside said first and second detector arrays; wherein saidfirst detector array is at least about ten percent smaller than saidsecond detector array.
 90. A positron emission tomography scanner as setforth in claim 89 first detector array is at least about thirty-threepercent smaller than said second detector array.
 91. A positron emissiontomography scanner as set forth in claim 90 first detector array is atleast about sixty-seven percent smaller than said second detector array.92. A positron emission tomography scanner as set forth in claim 89wherein said stage is located to center the region of interest of theobject at a point concentric with the first and second detector arrays.93. A method of increasing resolution of an image of a region ofinterest of an object provided by a positron emission tomography scannercomprising first and second circular concentric detector arrays, saidfirst detector array being at least about ten percent smaller than saidsecond detector array, said method comprising the steps of: centeringthe region of interest of the object at a point inside the first andsecond detector arrays; and scanning the object with the scanner.
 94. Ina positron emission tomography scanner having a primary positronemission tomography scanner for providing an image of a region of anobject, said primary scanner having opposing planar detector arraysspaced by a distance, each of said planar detector arrays including atleast one detector having an intrinsic spatial resolution, animprovement comprising a positron emission tomography detector modulefor providing an image of a region of an object said detector modulecomprising an accessory planar detector array including at least oneaccessory detector having an intrinsic spatial resolution at least asgood as the intrinsic spatial resolution of said primary scannerdetector, said accessory planar detector array of said detector modulebeing positioned inside an outer boundary defined by said opposingplanar detector arrays of the primary scanner.
 95. An improvement as setforth in claim 94 wherein said accessory detector has a width less thana width of said primary scanner detectors.
 96. An improvement as setforth in claim 95 wherein said accessory detector has a length less thana length of said primary scanner detectors.
 97. An improvement as setforth in claim 94 wherein said accessory detector has a length less thana length of said primary scanner detectors.
 98. In a positron emissiontomography scanner having a primary positron emission tomography scannerfor providing an image of a region of an object, said primary scannerhaving a primary detector array including a plurality of detectorsdefining an outer boundary, each of said detectors having an intrinsicspatial resolution, the improvement comprising a positron emissiontomography detector module for providing an image of a region of anobject, said detector module comprising an accessory detector arrayincluding a plurality of detectors, at least one detector of saidaccessory detector array having an intrinsic spatial resolution at leastas good as the intrinsic spatial resolution of each of said primaryscanner detectors, said accessory detector array being positioned insidethe outer boundary of said primary detector array.
 99. An improvement asset forth in claim 98 wherein said accessory detector has a width lessthan a width of said primary scanner detectors.
 100. An improvement asset forth in claim 99 wherein said accessory detector has a length lessthan a length of said primary scanner detectors.
 101. An improvement asset forth in claim 98 wherein said accessory detector has a length lessthan a length of said primary scanner detectors.
 102. An improvement asset forth in claim 98 wherein said accessory detector array issemi-cylindrical.
 103. An improvement as set forth in claim 98 whereinsaid accessory detector array is cylindrical.